Two quick questions:

1. Shouldn't addr https://github.com/withoutboats/uring-sys/blob/73cd794de0ccf9891098b7b3456d480ea5677ebc/src/lib.rs#L251 be declared as *mut libc::c_void? *const libc::c_void is good for everything except
   IORING_OP_READ_FIXED, and indeed it is declared as *mut libc::c_void here https://github.com/withoutboats/uring-sys/blob/73cd794de0ccf9891098b7b3456d480ea5677ebc/src/lib.rs#L269
2. Passing libc::off_t as offset in prep_{read, write} functions, for example here https://github.com/withoutboats/uring-sys/blob/73cd794de0ccf9891098b7b3456d480ea5677ebc/src/lib.rs#L290 seems strange for me, because at least on my machine this is i64 and we have u64 here https://github.com/withoutboats/uring-sys/blob/73cd794de0ccf9891098b7b3456d480ea5677ebc/src/lib.rs#L64

Agreed these are odd, but they're the types liburing exposes. I just checked: io_uring_prep_rw takes the addr as const void *addr and the offsets are passed as off_t.

And yea, everything but read_fixed also takes a const pointer, I assume because only read_fixed writes directly into that buffer.

Btw: do you plan to use prep helpers in iou?

Yes, to let liburing handle the subtleties of preparing IO events properly.